I used to own a 1988 Offshore. With sprayed on Teflon bottom paint (very smooth) boat would do 44mph as per GPS with 1988 225 Evinrude spinning a 15X17 aluminum prop. This was a notched transom boat, with no hardtop. This was lightly loaded with 50 gallons of fuel in flat water.
I am no expert, but in almost every performance test I haver read, when comparing the same boat with twins to a single outboard the findings are similar. Here are my observations: Boat A. has 225 single, to achieve same top end boat needs about twin 125hp (if available). Boat B. has 250 single, to achieve same top end needs about twin 140hp (if available). This "need" for twins to have more total horsepower is due to added weight and added drag.
Most boat owners (especially Grady owners) do not operate a boat at full throttle all that often. However it is a a good measurement of overall boat performance. I had very few worries about heading offshore with my single. But given the choice on that boat (or my current 24' Explorer) I would prefer a set of new 140 or 150 hp. Not necessarily for the speed or the redndancy, but for handling and choices. You could run on one when slow trolling, docking would be greatly improved, and 'somtimes' fuel economy actually improves.
Sorry for the long post...